Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over adsorbent beds which preferentially adsorb a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated to a higher pressure during intervals of flow in a first direction through the adsorbent bed, and is reduced to a lower pressure during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. Enhanced separation performance is achieved in well known PSA cycles using steps for each adsorbent bed of cocurrent feed at the higher cycle pressure cocurrent initial blowdown, countercurrent final blowdown, countercurrent purge at the lower cycle pressure, and countercurrent pressurization. Conventional pressure swing adsorption processes make inefficient use of applied energy, because of irreversible expansion over the valves while switching the adsorbent beds between higher and lower pressures. As disclosed by Kiyonaga (U.S. Pat. No. 3,176,444), Wagner (U.S. Pat. No. 3,430,418) and Fuderer (U.S. Pat. No. 3,986,849), improved efficiency and product yield can be obtained with more than two adsorbent beds operating in parallel, by performing pressure equalization steps between the separate beds so that a first bed undergoing a pressure reduction step exchanges gas which typically has been substantially purified to a second bed undergoing a pressure increase step so that the working pressure of the first and second beds is equalized to a pressure intermediate between the high and low pressures of the cycle. Thus, a cocurrent blowdown step of one bed achieves a countercurrent pressurization step in another bed when the product ends of the beds are connected during a pressure equalization step. This pressure equalization technique achieves partial recovery of bed expansion energy, although irreversible expansion still takes place over a smaller pressure interval. With a greater number of beds, multiple pressure equalization steps can be achieved, although the valve logic and controls are often greatly complicated.
Devices with simplified logic for achieving pressure equalization steps between multiple adsorbent beds have been disclosed by Van Weenen (U.S. Pat. No. 4,469,494) and by Mattia (U.S. Pat. No. 4,452,612) using a rotary adsorbent bed assembly whose multiple elements sweep past fixed ports for feed admission, product delivery and pressure equalization.
The prior art also includes the following pressure swing adsorption devices with cyclically operated volume displacement means such as reciprocating pistons communicating with one or both ends of an adsorbent bed, to generate pressure changes internally and thus improve energy efficiency. Pressure swing adsorption devices with pistons only at the feed end of the adsorbent bed are disclosed by Broughton (U.S. Pat. No. 3,121,625), Wilson (U.S. Pat. No. 3,164,454), Rutan (U.S. Pat. No. 3,236,028), Eriksson (U.S. Pat. No. 4,169,715) and Izumi et al (U.S. Pat. No. 4,948,401). Keller (U.S. Pat. No. 4,354,859) and my U.S. Pat. Nos. 4,702,903, 4,801,308, 4,816,121, 4,968,329, 5,082,473 and 5,096,469) have disclosed pressure swing adsorption devices with cyclic volume displacement means, operating at the same frequency and in general different phase, communicating with both ends of an adsorbent bed.
My U.S. Pat. No. 4,702,903 uses reciprocating volume displacement means coupled to an adsorbent bed, with a temperature gradient imposed on the adsorbent bed which also serves as a thermal regenerator, so that heat may be applied to assist driving the process through a regenerative thermodynamic cycle analogous to the Stirling cycle. Thus, heat is applied directly as an energy source to perform gas separations. Extensions of this principle are further developed in my U.S. Pat. No. 4,816,121 concerned with separation of chemically reactive gases, my U.S. Pat. No. 4,968,329 with scavenging valve logic means to provide large exchanges of fresh feed gas for depleted feed gas, and my U.S. Pat. No. 5,096,469) with inertial energy exchange between gas separation modules.
The present invention is related to concurrently filed U.S. patent application "Pressure Swing Adsorption Apparatus".